1. Field of the Invention
The present invention relates to imaging systems, and, more particularly, to a method of controlling paper position in a printing apparatus, such as an electrophotographic machine.
2. Description of the Related Art
In known electrophotographic machines, a sheet of paper is picked from an input device, such as a paper tray, prior to the start of the imaging process on a developing unit. As the paper moves through the paper path at a constant speed, the leading edge of the paper eventually trips a paper path sensor which signals the machine to commence imaging onto the developing unit. The image is laid down on a single rotating photoconductive drum in the developing unit. Eventually, the image on the drum is transferred to a transfer medium and then onto a sheet of paper which contacts the transfer medium at a precise point in time in order to establish a desired top writing line margin. This process is repeated for each individual sheet of paper, allowing the machine the luxury of waiting until each page is at a known location in the paper path before beginning the imaging process. Since the paper is picked prior to imaging, thereby allowing the electrophotographic process to wait for the paper to arrive at a certain location before commencing imaging, the margin above the top writing line on the paper can be accurately controlled.
It may be desirable or necessary to design an electrophotographic machine such that imaging is begun on a developing unit before the sheet of paper is even picked from the input device. This requirement may be due to size limitations on the printer which reduce the maximum length of the paper path. It may also be due to the use of multiple developing units, each transferring an image of a respective color onto a same location on the transfer medium. That is, after an image is first scanned onto a photoconductive drum for a first color, the image is transferred onto an intermediate transfer medium belt. The belt then moves over to receive an image of a second color from a second photoconductive drum. The second image is received on top of and overlaps the first image. This process repeats for each of the photoconductive drums, and the completed composite image eventually reaches the paper transfer nip where it is transferred from the intermediate transfer medium belt onto the paper. Since additional time is required for transferring a separate image from each developing unit sequentially, it may be necessary to begin imaging on at least a first of the developing units before the paper is picked from the input device.
A problem is that, after being picked, the paper can have several millimeters of error in the paper path. This error can be due to, for example, irregular pick times, paper stack height, and/or slippage. Thus, if the movement of the paper along the paper path continues at a constant speed, the paper will arrive at the toner transfer nip at an incorrect time. This results in a very unpredictable and inconsistent top writing line margin.
What is needed in the art is a method of media-to-image registration that eliminates errors in the size of a top writing line margin that are caused by commencing an imaging process onto an intermediate transfer medium before a sheet of paper is picked from an input device.